Conventionally, a pump used as a power source of a powered device has a relief valve in order to prevent an extraordinary increase of discharge presure. When the discharge presure of the pump reaches a predetermined presure, the relief valve opens a relief path so that discharged fuel can flow through the relief path toward the suction portion of the pump.
It has been also suggested to use variable-capacity pumps in order to prevent an extraordinarily high pressure. However, these conventional variable-capacity pumps vary their capacity in accordance with the rotation speed of the pump or the amount of discharged fluid. No variable capacity pump which varies its capacity in accordance with the discharge presure has heretofore been developed, to the present inventors' knowledge.